1. Field of the Invention
The invention relates to a super-high pressure discharge lamp of the short arc type in which the mercury vapor pressure during operation is at least 150 atm. The invention relates especially to a super-high pressure discharge lamp of the short arc type which is used as the backlight of a liquid crystal display device, a projection device, such as a DLP (digital light processor), or the like, in which a DMD (digital mirror device) is used.
2. Description of Related Art
In a projector device of the projection type, there is a demand for uniform illumination of the images onto a rectangular screen with sufficient color reproduction. The light source is thus a metal halide lamp which is filled with mercury and a metal halide. Furthermore, recently smaller and smaller metal halide lamps, and more and more often spot light sources, have been produced, and lamps with extremely small distances between the electrodes have been used in practice.
Against this background, instead of metal halide lamps, lamps with an extremely high mercury vapor pressure, for example, with 150 atm, have recently been proposed. Here, the increased mercury vapor pressure suppresses broadening of the arc (the arc is compressed) and a major increase of the light intensity is desired.
Such a super-high pressure discharge lamp is disclosed, for example, in Japanese patent disclosure document HEI 2-148561 (U.S. Pat. No. 5,109,181) and in Japanese patent disclosure document HEI 6-52830 (U.S. Pat. No. 5,497,049).
In such a super-high pressure discharge lamp, the pressure in the arc tube during operation is extremely high. In the side tube portions which extend from opposite sides of the arc tube portion, it is therefore necessary to arrange the silica glass comprising these side tube portions, the electrodes and the metal foils for current supply in a sufficient amount and moreover, fixed directly tightly adjoining one another. The reason for this is that the added gas emerges or cracks form when this adhesive property is not adequate.
Therefore, in the process of hermetic sealing of the side tube portions, for example, at a high temperature of 2000° C., the silica glass is heated, and in this state, the viscous silica glass is gradually contracted or the silica glass is subjected to a pinch seal. This improves the adhesive property of the side tube portions.
However, if the silica glass bums at an overly high temperature, the disadvantage arises that, after completion of the discharge lamp, the side tube portions are easily damaged, even if the adhesion of the silica glass to the electrodes or metal foils improves. This disadvantage is explained in detail below.
In the stage after heat treatment in which the temperature of the side tube portions gradually drops, due to the difference of relative amounts of expansion, as a result of the difference between the coefficient of thermal expansion of the material (tungsten) comprising the electrodes and the coefficient of thermal expansion of the material (silica glass) comprising the side tube portions, cracks form in the contact areas of the two. The cracks are extremely small. However, the cracks grow during lamp operation and in the super-high pressure state during operation. It can be imagined that this would cause damage to the discharge lamp.
To eliminate this defect, the arrangement shown in FIG. 6 is proposed. In the figure, the side tube portions 3 are connected to the arc tube portion 2 of a discharge lamp 1. In the side tube portions 3, the electrodes 6, 7 within the arc tube portion 2 are each connected to a metal foil 8. Electrode rods 6a, 7a are installed in the side tube portions 3 and are each wound with a coil component 10. In this arrangement, the stress exerted on the silica glass a result of thermal expansion of the electrode (rods) is relieved by the coil components 10 which are wound around the electrode rods. This arrangement is described, for example, in Japanese patent disclosure document HEI 11-176385.
However, even if this arrangement reduces the thermal expansion of the electrodes, in reality, in the vicinities of the electrodes 6,7, the electrode rods 6a, 7a and the coil components 10, cracks remain. These cracks are admittedly very small; but, there are cases in which they often lead to damage to the side tube portions 3 in the case of a mercury vapor pressure of the arc tube portion 2 of roughly 150 atm. In recent years, there has been a demand for a very high mercury vapor pressure of 200 atm or 300 atm. At such a high mercury vapor pressure, crack growth is accelerated during lamp operation. This results in the disadvantage that the side tube portions 3 are conspicuously damaged. This means that cracks during lamp operation at a high mercury vapor pressure gradually become large, even if they are extremely small at the start. It can be stated that eliminating this disadvantage is a new technical object which is never present in a mercury lamp with a vapor pressure from roughly 50 atm to 100 atm during operation.